Injecting a bacteria into mosquitoes can block them from transmitting the dengue virus and help control the spread of a disease that kills 20,000 annually in more than 100 countries, scientists said. In two papers published in the journal Nature on Thursday, researchers in Australia showed how female mosquitoes infected with the Wolbachia bacteria passed the bug easily to their offspring, making them all dengue-free. They said such infected mosquitoes should be released into the wild, so that the spread of dengue to people may be reduced.